A magnetoresistance device is a type of memory device in which data can be stored as an alterable orientation of magnetization. As one example, a tunneling magnetoresistance (TMR) device can include a reference layer (also referred to as a pinning film or pinned layer) that includes a pinned orientation of magnetization that is fixed in a predetermined orientation, a data layer that includes an alterable orientation of magnetization that can be altered by an external magnetic field, and a thin tunnel barrier layer that separates the data layer from the reference layer.
A state of the data stored in the data layer is determined by an orientation of the alterable orientation of magnetization relative to the pinned orientation of magnetization. For example, if the alterable orientation of magnetization is oriented in the same direction as the pinned orientation of magnetization (e.g. parallel), then a logic “1” is stored in the data layer. On the other hand, if the alterable orientation of magnetization is oriented in an opposite direction as the pinned orientation of magnetization (e.g. anti-parallel), then a logic “0” is stored in the data layer.
For a TMR device, the state of the data stored in the data layer is determined by measuring or sensing a tunneling resistance across the data and reference layers. One value of resistance is indicative of the logic “1” and a different value of resistance is indicative of the logic “0”. It is desirable to have the value of resistance for the logic “1” be as far apart as possible from the value of resistance for the logic “0”. The further apart those two values are, the higher a signal-to-noise ratio ΔR/R of the TMR device. The ΔR is a change in resistance from a logic “1” or a logic “0” or vice-versa and R is a lower of the resistance values for a logic “1” to a logic “0”. A high signal-to-noise ratio allows for accurate sensing of the data in the data layer during a read operation to the TMR device. Accurate sensing is a necessity if the TMR device is to be used for data storage (e.g. as a MRAM device). A low signal-to-noise ratio is undesirable because the value of resistance for the logic “0” is not different enough from the value of resistance for the logic “1”; therefore, the state of the data cannot be accurately determined and the TMR device will not be suitable as a memory device for data storage.
The signal-to-noise ratio ΔR/R can be increased by depositing a thin layer of a polarizing material, such as iron (Fe), cobalt (Co), or cobalt and iron (CoFe), at an interface between the tunnel barrier layer and the data and references layers. The layer of the polarizing material must be very thin (e.g. only a few monolayers thick) and must uniformly cover the surface it is deposited on. Prior deposition processes include standard sputtering, atomic layer deposition (ALD), and molecular beam epitaxy (MBE).
Disadvantages to prior sputtering deposition systems include a non-uniform coverage of the polarizing material on the surface it is deposited on. Metals (e.g. Fe, Co, or CoFe) tend to form island growth, and then coalesce into a continuous and non-uniform film. Additionally, depositing a uniform layer with a thickness of a few monolayers is not possible using the prior sputtering deposition systems. In some applications (e.g. MRAM) it is desirable to limit the thickness of the polarizing material, since highly polarized materials tend to have a high saturation magnetization (Ms). A high Ms can contribute to a high ferromagnetic Ne'el coupling, a high antiferromagnetic demagnetization coupling in the antiferromagnetic layer, and a high coercivity in the ferromagnetic data layer. Accordingly, a method of depositing a few monolayers (e.g about 5.0 monolayers or less) of the highly polarized material is desired.
ALD is another prior method for depositing layers of material that are only a few monolayer thick. ALD can create uniform layers with very controllable thickness; however, the deposited layer is conformal to the underlying topography of the underlying surface. Therefore, a surface roughness or defects in the topography can result in a non-uniform layer of the polarizing material. Another disadvantage of ALD is that it is a reactive deposition method, not a direct deposition method. For some materials, ALD requires a water (H2O) precursor, which is destructive to the ferromagnetic materials in a TMR device (i.e. the water causes corrosion).
Finally, MBE is capable of true atomic layer growth of layers that are a few monolayers thick; however, MBE is a prohibitively expensive process that is not economically viable for the mass production of semiconductor devices. Moreover, a range of materials that are compatible with the MBE process is limited.
Consequently, there exists a need for a method of fabricating a thin and uniform polarizing layer on an interface surface. There is also a need for a method of fabricating a polarizing layer on an interface surface that also reduces a surface roughness of the interface surface.